/* MWC1616 return value made by concatenating two 16-bit multiply-with-carry generators. The two generators have the form x(n)=a*x(n-1)+carry mod 2^16 and y(n)=b*y(n-1)+carry mod 2^16, in this case a and b are choosen as 18000 and 30903 Some other good choices for a and b 18000 18030 18273 18513 18879 19074 19098 19164 19215 19584 19599 19950 20088 20508 20544 20664 20814 20970 21153 21243 21423 21723 21954 22125 22188 22293 22860 22938 22965 22974 23109 23124 23163 23208 23508 23520 23553 23658 23865 24114 24219 24660 24699 24864 24948 25023 25308 25443 26004 26088 26154 26550 26679 26838 27183 27258 27753 27795 27810 27834 27960 28320 28380 28689 28710 28794 28854 28959 28980 29013 29379 29889 30135 30345 30459 30714 30903 30963 31059 31083 The `carry' c works as follows: If a and x are 16-bit and c at most 14 bits, then forming a*x+c produces an at-most 31- bit result. That result mod 2^16 (the rightmost 16 bits) is the new x and the topmost 16 bits the new carry c. The sequ- ence of resulting x's has period the order of 2^16 in the group of residues relatively prime to m=a*2^16-1, which will be a*2^15-1 for the multipliers suggested here. You will be prompted to choose a and b and two seeds. Output is a 32-bit integer, the pair x,y side by side. This multiply-with-carry generator is best done in assembler, where it takes about 200 nanosecs with a Pentium 120. A Fort- ran version takes about 300 ns. It seems to pass all tests and is highly recommended for speed and simplicity. */ #define ulong unsigned long static ulong mwc1616_x = 1; static ulong mwc1616_y = 2; void seed_rand_mwc1616(ulong seed) { mwc1616_x = seed | 1; /* Can't seed with 0 */ mwc1616_y = seed | 2; } ulong rand_mwc1616(void) { mwc1616_x = 18000 * (mwc1616_x & 0xffff) + (mwc1616_x >> 16); mwc1616_y = 30903 * (mwc1616_y & 0xffff) + (mwc1616_y >> 16); return (mwc1616_x<<16)+(mwc1616_y&0xffff); }